Proton exchange membrane (PEM) fuel cell stacks are commonly configured having a plurality of fuel cell elements in a stacked configuration. The fuel cell elements commonly include a pair of PEM elements separated by a bipolar plate. Bipolar plates are commonly constructed having a pair of plates joined by adhesive seal, by brazing, and by welding.
Bipolar plates are commonly constructed with an upper plate and a lower plate. Both the upper plate and the lower plate include a plurality of reactant gas passages or grooves formed in the outer facing surfaces of the two plates. A plurality of coolant channels or portions of coolant channels are formed on the inner facing surfaces of the bipolar plates. When the two plate pairs are joined, the completed coolant channels are formed. Each of the coolant channels is normally separated by a plurality of lands. Electrical current is withdrawn from each of the bipolar plate assemblies; therefore, it is necessary that each of the pairs of plates forming a bipolar plate assembly be mechanically as well as electrically joined.
It is known to coat each of the interfacing surfaces (including the coolant channels) with an electrically conductive metal. Gold has been used for this purpose. The two plate halves can then be butted having the gold plated surfaces across the plurality of lands joined to electrically connect the bipolar plate assembly. A disadvantage of using gold for the plating material is both the thickness and the cost of the gold required for the plating. The gold is plated in approximately 25 μm thicknesses. If pressure alone is used to join the plate pairs to form a bipolar plate assembly, cost of the fuel cell stack is increased due to the cost of gold and the thickness of the gold plating material required to provide adequate land-to-land contact surface. Note that the reactants (humidified air) and coolants used in a typical PEM fuel cell stack can have a detrimental impact on plating materials used to plate the metallic surfaces of the plate pairs.
Uncoated metallic plates can also be used to form bipolar plate assemblies. The disadvantage of directly abutting uncoated metal plate pairs to form the bipolar plate assembly is that an oxidation layer quickly forms over the surfaces of each of the plate pairs before joining, which later inhibits electrical conductivity between the joined plate pairs.
It is therefore desirable to provide a bipolar plate assembly which reduces the cost associated with gold used as a plating material and which solves the potential problem of reactant gas and coolant contact with the plated surfaces adjacent to the coolant channels between the plate halves of a bipolar plate assembly.